Human_Histology

188 Textbook of Human Histology

PLATE 15.7: Ileum (Peyers Patch)

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Duodenum. A. As seen in drawing; B. Photomicrograph.

Courtesy: Atlas of Histopathology. Ist Edition. Ivan Damjanov. Jaypee Brothers. 2012. p 117

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Jejunum (Plate 15.5)

The proximal part of the jejunum shows significant differences in structure from the terminal part of the ileum. The changes take place gradually in proceeding caudally along the small intestine, there being no hard and fast line of distinction between the jejunum and the ileum. As compared to the ileum the jejunum has the following features:

A larger diameter

A thicker wall

Larger and more numerous circular folds

Larger villi

Fewer solitary lymphatic follicles. Aggreated lymphatic follicles are absent in the proximal jejunum, and small in the distal jejunum

Greater vascularity

Ileum

The villi are thin and slender in the region of ileum. The submucosa contains the Peyer's patches (Plate 15.7). M-cells are found overlying the lymphoid follicles.

Pathological Correlation

Crohn’s Disease or Regional Enteritis is an idiopathic chronic! most commonly the segment of terminal ileum and/or colon, though any part of the gastrointestinal tract may be involved.

Coeliac Sprue is the most important cause of primary malabsorption occurring in temperate climates. The condition is! and thence diminished absorptive surface area. The condition occurs in 2 forms:

Childhood form, seen in infants and children and is commonly referred to as coeliac disease.

Adult form, seen in adolescents and early adult life and used to be called idiopathic steatorrhea.

THE LARGE INTESTINE (COLON)

It consists of the caecum, appendix, colon, rectum, and anal canal. The main functions of the large intestine are absorption of water and conversion of the liquid, undigested material into solid faeces. It harbors some nonpathogenic

Chapter 15 Digestive System: Esophagus, Stomach, and Intestines 189

Fig. 15.13: Basic features of the structure
	Fig. 15.14: Segment of the colon (Schematic representation)
intestine (Schematic representation)

bacteria that produce vitamin B12 and vitamin K. The former is necessary for hemopoiesis and the latter for coagulation of blood.

THE COLON

The structure of the colon conforms to the general description of the structure of the gut. The following additional points may be noted (Fig. 15.13 and Plate 15.8).

Mucous Membrane

The mucous membrane of the colon shows numerous crescent-shaped folds. There are no villi. The mucosa shows numerous closely arranged tubular glands or crypts similar to those in the small intestine. The mucosal surface, and the glands, are lined by an epithelium made up predominantly of columnar cells. Their main function is to absorb excess water and electrolytes from intestinal contents.

Many columnar cells secrete mucous and antibodies (IgA). The antibodies provide protection against pathogenic organisms. Numerous goblet cells are present, their number increasing in proceeding caudally. The mucous secreted by them serves as a lubricant that facilitates the passage of semisolid contents through the colon. Paneth cells are not present. Some endocrine cells, and some stem cells, are seen.

The epithelium overlying solitary lymphatic follicles (present in the lamina propria) contains M-cells similar to those described in the small intestine. Scattered cells

bearing tufts of long microvilli are also seen. They are probably sensory cells.

Submucosa

The submucosa often contains fat cells. Some cells that contain PAS-positive granules, termed muciphages, are also present. These are most numerous in the rectum.

Muscularis Externa

The longitudinal layer of muscle is unusual. Most of the fibers in it are collected to form three thick bands, the taenia coli (Fig. 15.14). A thin layer of longitudinal fibers is present in the intervals between the taenia. The taenia are shorter in length than other layers of the wall of the colon. This results in the production of sacculations on the wall of the colon.

Serosa

The serous layer is missing over the posterior aspect of the ascending and descending colon. In many situations the peritoneum forms small pouch-like processes that are filled with fat. These yellow masses are called the appendices epiploicae.

THE VERMIFORM APPENDIX

The appendix is the narrowest part of the gut. The structure of the vermiform appendix resembles that of the colon with the following differences (Plate 15.9):

The crypts are poorly formed.

The longitudinal muscle coat is complete and equally thick all round. Taenia coli are not present.

190 Textbook of Human Histology

PLATE 15.8: Large Intestine

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Large Intestine. A. As seen in drawing; B. Photomicrograph

Courtesy: Atlas of Histopathology. Ist Edition. Ivan Damjanov. Jaypee Brothers. 2012. p 122

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The submucosa contains abundant lymphoid tissue that may completely fill the submucosa. The lymphoid tissue is not present at birth. It gradually increases and is best seen in children about 10 years old. Subsequently, there is progressive reduction in quantity of lymphoid tissue.

Peritoneum covers the front and sides of the upper onethird of the rectum; and only the front of the middle third. The rest of the rectum is devoid of a serous covering.

There are no appendices epiploicae.

THE ANAL CANAL

THE RECTUM

The structure of the rectum is similar to that of the colon except for the following:

A continuous coat of longitudinal muscle is present. There are no taenia.

The anal canal is about 4-cm long. The upper 3 cm are lined by mucous membrane, and the lower 1 cm by skin.

Mucosa

The mucous membrane of the upper 15 mm of the canal is lined by columnar epithelium. The mucous membrane

Chapter 15 Digestive System: Esophagus, Stomach, and Intestines 191

PLATE 15.9: Vermiform Appendix

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of this part shows 6 to 12 longitudinal folds that are called the anal columns. The lower ends of the anal columns are united to each other by short transverse folds called the anal valves. The anal valves together form a transverse line that runs all round the anal canal, this is the pectinate line (Fig. 15.15).

The mucous membrane of the next 15 mm of the rectum is lined by non-keratinized stratified squamous epithelium. This region does not have anal columns. The mucosa has a bluish appearance because of the presence of a dense venous plexus between it and the muscle coat. This region is called the pecten or transitional zone. The lower limit of the pecten forms the white line (of Hilton).

The lowest 8 to 10 mm of the anal canal are lined by true skin in which hair follicles, sebaceous glands and sweat glands are present.

Above each anal valve there is a depression called the anal sinus. Atypical (apocrine) sweat glands open into each sinus. They are called the anal (or circumanal) glands.

Submucosa

Prominent venous plexuses are present in the submucosa of the anal canal. The internal hemorrhoidal plexus lies above the level of the pectinate line, while the external hemorrhoidal plexus lies near the lower end of the canal.

192 Textbook of Human Histology

Fig. 15.15: Some features in the interior of the anal canal

(Schematic representation)

Muscularis Externa

The anal canal is surrounded by circular and longitudinal layers of muscle continuous with those of the rectum. The circular muscle is thickened to form the internal anal sphincter. Outside the layer of smooth muscle, there is the external anal sphincter that is made up of striated muscle. For further details of the anal musculature see a book on gross anatomy.

Clinical Correlation

Ulcerative colitis is an idiopathic form of acute and chronic submucosa of the rectum and descending colon, though sometimes it may involve the entire length of the large bowel.

" acute appendicitis, is the most common acute abdominal condition confronting the surgeon. The condition is seen more commonly in older children and young adults, and is uncommon at the extremes# $

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% with low bulk or cellulose and high protein intake more often causes appendicitis.

Hemorrhoids or piles are the varicosities of the hemorrhoidal veins. They are called “internal piles” if dilatation is of superior hemorrhoidal plexus covered over by mucous membrane, and “external piles” if they involve inferior hemorrhoidal plexus covered over by the skin. They are common lesions in elderly and pregnant women. They commonly result from increased venous pressure.

Added Information

The Endocrine Cells of the Gut

The lining epithelium of the stomach, and of the small and large intestines, contains scattered cells that have+ ' ' therefore, termed cells. The granules also show5& 5 also found in the nervous system where they function as neurotransmitters. They also act as hormones. This action 67 8 now grouped together under the term gastro-entero- pancreatic endocrine system. Some of the cell types recognized, and their secretory products are given in the 9< =

Some features of this system are similar to those of amine producing cells in other organs. All these are included under the term APUD cell system.

Table 15.2: Different types of endocrine cells in the gut and in the pancreas

			Distribution (+ = present)
Cell type	Secretory products	Stomach	Small intestine	Large intestine	Pancreas
D1	7	+	+	+	+
D	Somatostatin	+	+		+
EC1	<- ? "		+	+
L	Enteroglucagon		+	+
G	Gastric encephalin	+	+
EC2	<- ? 5		+
S	Secretin		+
I	Cholecystokinin pancreozymin		+
K	@		+
N	Neurokinin		+
ECn	<- ? '	+
B	Insulin				+
A	Glucagon	?	?		+
PP	Pancreatic polypeptide		?		+

Hepatobiliary System and Pancreas

The hepatobiliary system comprises of liver, gallbladder and extrahepatic ducts.

THE LIVER

Liver is the largest gland of the body situated mainly in the right hypochondrium, below the right dome of diaphragm in the abdomen.

The liver may be regarded as a modified exocrine gland that also has other functions. The liver substance is divisible into a large number of large lobes, each of which consists of numerous lobules (Plate 16.1).

MICROSCOPIC FEATURES

Glisson’s Capsule

The liver is covered by a capsule (Glisson’s capsule) made up of connective tissue. This connective tissue extends into the liver substance through the portal canals (mentioned above) where it surrounds the portal triads. The sinusoids are surrounded by reticular fibers. Connective tissue does not intervene between adjoining liver cells.

Hepatic Lobules

In sections through the liver, the substance of the organ appears to be made up of hexagonal areas that constitute the hepatic lobules (Fig. 16.1). In some species (e.g. the pig) the lobules are distinctly demarcated by connective tissue septa, but in the human liver the connective tissue is scanty and the lobules often appear to merge with one another.

In transverse sections, each lobule appears to be made up of cords of liver cells that are separated by sinusoids. However, the cells are really arranged in the form of plates (one cell thick) that branch and anastomose with one another to form a network. Spaces within the network are occupied by sinusoids.

Portal Canal

Along the periphery of each lobule there are angular intervals filled by connective tissue. These intervals are

called portal canals, the “canals” forming a connective tissue network permeating the entire liver substance.

Each “canal” contains:

A branch of the portal vein

A branch of the hepatic artery

An interlobular bile duct

These three structures collectively form a portal

triad (Fig. 16.2). Blood from the branch of the portal vein, and from the branch of the hepatic artery, enters the sinusoids at the periphery of the lobule and passes toward its center. Here the sinusoids open into a central vein that occupies the center of the lobule. The central vein drains into hepatic veins (which leave the liver to end in the inferior vena cava).

Blood vessels and hepatic ducts present in portal canals are surrounded by a narrow interval called the space of Mall.

Portal Lobules

The vessels in a portal triad usually give branches to parts of three adjoining lobules. The area of liver tissue (comprising parts of three hepatic lobules) supplied by one branch of the portal vein is regarded by many authorities as the true functional unit of liver tissue, and is referred to as a portal lobule (Fig. 16.3).

A still smaller unit, the portal acinus has also been described. It consists of a diamond shaped area of liver tissue supplied by one hepatic arteriole (Fig. 16.4) running along the line of junction of two hepatic lobules. Two central veins lie at the ends of the acinus.

Duct System

Bile secreted by liver cells is poured into bile canaliculi. These canaliculi have no walls of their own. They are merely spaces present between plasma membranes of adjacent liver cells. The canaliculi form hexagonal networks around the liver cells. At the periphery of a lobule the canaliculi become continuous with delicate intralobular ductules, which in turn become continuous with larger interlobular ductules of portal triads.

194 Textbook of Human Histology

PLATE 16.1: Liver (Panoramic View)

	The panoramic view of liver shows many hexagonal areas

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Liver (panoramic view). A. As seen in drawing;

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Courtesy: Atlas of Histopathology. Ist Edition. Ivan

Damjanov. Jaypee Brothers. 2012. p 142

The interlobular ductules are lined by cuboidal epithelium. Some smooth muscle is present in the walls of larger ducts.

Hepatocytes

Liver is made up, predominantly, of liver cells or hepatocytes. Each hepatocyte is a large cell with a round openfaced nucleus, with prominent nucleoli (Plate 16.2).

The cytoplasm of liver cells contains numerous mitochondria, abundant rough and smooth endoplasmic reticulum, a well developed Golgi complex, lysosomes, and vacuoles containing various enzymes. Numerous free ribosomes are present. These features are to be correlated with the high metabolic activity of liver cells. Stored glycogen, lipids, and iron (as crystals of ferratin and hemosiderin) are usually present. Glycogen is often present in relation to

Chapter 16 Hepatobiliary System and Pancreas 195

Fig. 16.1: Classic liver lobule (Schematic representation)

Fig. 16.2: Portal triad (Schematic representation)

Fig. 16.3: Scheme to show the concept of portal lobules (pink).	Fig. 16.4: Scheme to show the concept of portal acini (pink)
Hepatic lobules are shaded green. Note that the portal lobule is
made up of parts of three hepatic lobules

smooth endoplasmic reticulum. Many hepatocytes show two nuclei; or a single polyploid nucleus.

Liver cells are arranged in the form of anastomosing plates, one cell thick; and that the plates form a network in the spaces of which sinusoids lie (Fig. 16.5). In this way each liver cell has a sinusoid on two sides. The sinusoids are lined by an endothelium in which there are numerous pores (fenestrae). A basement membrane is not seen. Interspersed amongst the endothelial cells there are hepatic macrophages (Kupffer cells).

The surface of a hepatocyte can show three kinds of specialization (Fig. 16.6):

Sinusoidal surface: The cell surfaces adjoining sinusoids bears microvilli that project into the space of Disse. The cell surface here also shows many coated pits that are concerned with exocytosis. Both these features are to be associated with active transfer of materials from sinusoids to hepatocytes, and vice versa. About 70% of the surface of hepatocytes is of this type.

Canalicular surface: Such areas of cell membrane bear longitudinal depressions that are apposed to similar depressions on neighboring hepatocytes, to form the wall of a bile canaliculus. Irregular microvilli project into the canaliculus. On either side of the canaliculus, the cell membranes of adjoining cells are united by junctional complexes. About 15% of the hepatocyte surface is canalicular.

Intercellular surface: These are areas of cell surface where adjacent hepatocytes are united to each other just as in typical cells. Communicating junctions allow exchanges between the cells. About 15% of the hepatocyte surface is intercellular.

Space of Disse

The surface of the liver cell is separated from the endothelial lining of the sinusoid by a narrow perisinusoidal space (of Disse) (Fig. 16.7). Microvilli, present on the liver cells,

196 Textbook of Human Histology

PLATE 16.2:

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Fig. 16.5: Relationship of bile capillaries to liver cells (Schematic representation)

extend into this space. As a result of these factors hepatocytes are brought into a very intimate relationship with the circulating blood. Some fat cells may also be seen in the space of Disse.

BILE

The exocrine secretion of the liver cells is called bile. Bile is poured out from liver cells into very delicate bile canaliculi that are present in intimate relationship to the cells. From the canaliculi bile drains into progressively larger ducts that end in the bile duct. This duct conveys bile into the duodenum where bile plays a role in digestion of fat.

Fig. 16.6: Three functional specializations of cell surface of a hepatocyte (Schematic representation)

Fig. 16.8: Scheme to show the presence of several branches of the portal vein around a hepatic lobule. The manner in that they open into sinusoids is shown. The intervals between the sinusoids are occupied by liver cells (Schematic representation)

BLOOD SUPPLY OF LIVER

In addition to deoxygenated blood reaching the liver through the portal vein (Fig. 16.8), the organ also receives oxygenated blood through the hepatic artery and its branches. The blood entering the liver from both these sources passes through the hepatic sinusoids and is collected by tributaries of hepatic veins. One such tributary runs through the center of each lobule of the liver where it is called the central vein (Fig. 16.8).

Branches of the hepatic artery, the portal vein, and the hepatic ducts, travel together through the liver. The tributaries of hepatic veins follow a separate course.

Chapter 16 Hepatobiliary System and Pancreas 197

Fig. 16.7: Space of Disse and bile canaliculus

(Schematic representation)

FUNCTIONS OF LIVER

The liver performs numerous functions. Some of these are as follows:

The liver acts as an exocrine gland for the secretion of bile. However, the architecture of the liver has greater resemblance to that of an endocrine gland, the cells being in intimate relationship to blood in sinusoids. This is to be correlated with the fact that liver cells take up numerous substances from the blood, and also pour many substances back into it.

The liver plays a prominent role in metabolism of carbohydrates, proteins and fats. Metabolic functions include synthesis of plasma proteins fibrinogen and prothrombin, and the regulation of blood glucose and lipids.

The liver acts as a store for various substances including glucose (as glycogen), lipids, vitamins, and iron. When necessary, the liver can convert lipids and amino acids into glucose (gluconeogenesis).

The liver plays a protective role by detoxifying substances (including drugs and alcohol). Removal of bile pigments from blood (and their excretion through bile) is part of this process. Amino acids are deaminated to produce urea, which enters the bloodstream to be excreted through the kidneys. The macrophage cells (of Kupffer) lining the sinusoids of the liver have a role similar to that of other cells of the mononuclear phagocyte system. They are of particular importance as they are the first cells of this system that come in contact with materials absorbed through the gut. They also remove damaged erythrocytes from blood.

During fetal life the liver is the center for hemopoiesis.